This invention relates to plasma generation equipment and techniques, and it is particularly directed to a method and apparatus for the quick protection and recovery from load transients, and protecting circuitry in plasma generation equipment from transient current and voltage peaks originating in the plasma chamber.
It has long been understood in the field of plasma generation that, extremely short and high power load transients may occur. The design of plasma generators, therefore, must afford the generator circuitry protection from such destructive transients. Moreover, such a system must be capable of reacting extremely quickly in recognizing and recovering from such transients, as those transients may be extremely brief. It is similarly advantageous for such a system to guarantee that the control system associated with the plasma generator is informed each and every time a transient occurs. This would allow the control system of the plasma generator to restore output power to its pre-transient set-point as rapidly as possible.
In a typical RF plasma generator arrangement, a generator supplies forward power to the plasma chamber. A portion of the forward power is reflected back towards the generator, as reverse, or reflected power. Conditions may occur, however, that can cause transient peaks in the reflected power. These transient peaks can cause excessive current or voltage peaks in the generator. These peaks are unpredictable, and are of such amplitude that they pose a threat to the integrity of circuits within the plasma generation equipment. Detection, effective countermeasures, and quick recovery from such transient peaks, therefore, is an important aspect of the design of plasma generation equipment.
Typical plasma generator protection schemes, however, merely focus on the cut-off of the input power (and therefore the output) to the plasma generator in order to protect the generator circuitry from damage. As will be discussed below, this protection scheme suffers from several disadvantages.
FIG. 1 depicts a block diagram of a typical transient protection scheme. This protection circuitry works on the aforementioned principle of simply shutting down the drive to the power amplifier 150 upon detection of a transient. When the sensor 110 detects a transient, it will turn off the drive to the power amplifier 150 via the switch 160.
Although this design may operate to protect the power amplifier circuitry from damage due to transients in the plasma chamber, it suffers from several shortcomings. Most important of these shortcomings is that, due to the rapid rise and short duration of some transients, there is a substantial probability that the controller 120 will not be able to react quickly enough to acknowledge that a transient has occurred. Accordingly, the response time of the protection system, from the time the transient occurs to the time the protection system restores power output to its previous levels, may be unacceptably long.
Additionally, the inability of present systems to acknowledge all transients may have damaging effects on the plasma generator""s circuitry. For example, if the controller 120 does not react quickly enough to realize that a transient has occurred, the software will also be unaware that a transient has occurred. Thus, the control software 130 will only realize that the power output from the power amplifier 150 has either diminished, or has shut off completely, since the input power to the power amplifier 150 has been cut by sensor 110. Accordingly, and in reaction to the above situation, a cascade effect will occur that may damage the circuitry of the system. That is, the software 130, unaware that input power was cut-off due to the transient, will increase its own internal parameters in an attempt to produce more power out of the generator in order to meet its present power set-point. Thus, when the transient actually does subside and the sensor 110 restores input power to the power amplifier, the software""s 130 internal control parameters will have been driven to a very high level; much higher than before the transient occurred. This may result in a second transient, which may destroy the circuit elements of the plasma generator.
Furthermore, since there is no guarantee that the software will be capable of reacting to each and every transient, accurate reporting of these transients is impossible. Thus, although it would be desirable to report on the frequency of transients within the system, such reporting is not possible using a protection scheme that cannot guarantee recognition of virtually each and every transient.
Accordingly, although present cut-off protection schemes offer at least some level of protection to power amplifier circuitry from the occurrence of transients, they are lacking in several respects. Most importantly, they are incapable of reacting quickly enough to realize that a transient has even occurred, and will have an unacceptable delay time from the point the transient is detected, to the point where output power from the power amplifier is turned off. Additionally, since present detection systems are incapable of reliably detecting transients, they can react incorrectly to the protective cut-off and produce a second equally destructive transient. Also, they cannot provide accurate reports of the occurrence of the transients, which would greatly aid in the design and improvement of plasma generator and plasma chamber design.
It is thus an object of the present application to solve these and many other problems. This object, and many other features and advantages of this invention will become apparent from the ensuing description.